Security, predictability, climate change, safety – all of these are cited by the U.S. Department of Defense as motivating factors for its embrace of energy-efficient and renewable technologies. But a new report from the Pew Charitable Trusts puts just as much emphasis on another factor that might come as a surprise: cost.

The Defense Advanced Research Projects Agency (DARPA) is currently working on an initiative that would allow large drones to take off and land from the surface of small warships - rather than gigantic aircraft carriers.

Not all of the drone aircraft deployed by militaries are actually large flying machines. Case in point? The uber-mini Black Hornet, manufactured by Prox Dynamics, which is slightly reminiscent a military-style MeCam.